1. Field of the Invention
The present invention relates to an adhesive composition for use in flexible printed circuit boards, and more particularly, to an adhesive composition with higher glass transition temperature (Tg) and longer storage period, comprising an epoxy resin, an accelerator, a nitrile rubber having carboxyl groups, and an inorganic filler, to improve the application and operation processing of flexible printed circuit boards.
2. Description of Related Art
The flexible printed circuit board (FPC board), also called the deflectable print circuit board, can be assembled in three diminsions according to the space of the final product because of its flexibility. Nowadays, the trend of the high technology instrument such as a camera, a video camera, a hi-fi equipment, a disc player, a printer and a cellular phone, is towards light, thin, short and small. Thus, the flexible printed circuit board becomes one of the important components of the high technology instrument.
The structure of the flexible printed circuit board can be divided into five basic parts, which are a prepreg, a copper foil, a substrate, a surface treatment and a reinforcement The first three parts are the major components of the flexible printed circuit board and are combined together by an adhesive. Therefore, the quality of the flexible printed circuit board is depends on the heat resistance and the storage period of the adhesive. The effectiveness and stability of the adhesive is influenced by heat, ambient temperature and moisture content produced by the operation of the flexible priented board. Therefore, the trend of the adhesive is towards higher glass transition temperature and longer storage period at room temperature.
R.O.C Patent Publication No. 279255 (Application No. 84105059) discloses an epoxy resin composition applied for packing a semiconductor device. However, the above patent is applied for packing the semiconductor device but not for improving the flexible printed circuit board. Moreover, the above epoxy resin composition has to contain an internal releasing agent such as silicone oil, fluoride type surfactant, waxes, fluid paraffin and metallic salt of stearic acid for facilitating the mold release.
U.S. Pat. No. 5,162,140 discloses a flexible printed circuit board and an adhesive thereinto. However, the accelerator contained in the above adhesive is a salt of Lewis acid rather than an onium salt of hexafluoroantimonic acid. U.S. Pat. No. 5,260,130 also discloses an adhesive for a flexible printed circuit board, which must contain a curing agent and an accelerator. The accelerator is a borofluoride of zinc, tin or nickel rather than an onium salt of hexafluoroantimonic acid.
The above patents are not applied to improve the glass transition temperature or the storage period of the adhesive of the flexible printed circuit board.
The present invention discloses an adhesive composition in accordance with the need of the market of the flexible printed circuit board, comprising an epoxy resin, an accelerator, a nitnl rubber having carboxy groups and an inorganic filler. The present invention provides an adhesive composition with higher glass transition temperature (140xc2x0 C. to 170xc2x0 C.) and longer storage period at room temperature (more than 50 days) by using an onium salt of hexafluoroantimonic acid as its accelerator. Therefore, the poor application of the printed circuit board because of the unstability of the adhesive is avoided.
A major object of the present invention is to provide an adhesive composition applied to a flexible printed circuit board and having high transition temperature (Tg) and long storage period, comprising an epoxy resin, an accelerator such as an onium salt of hexafluoroantimonic acid, a nitrile rubber having carboxyl groups and an inorganic filler.
The present invention provides an adhesive composition applied for a flexible printed circuit board and having properties of heat-resistant, high transition temperature, long storage period at room temperature and rapid curing reaction, comprising:
(a) an epoxy resin;
(b) an accelerator having a structure of one of the following general formulas: 
wherein, E represents sulfur, nitrogen or phosphorus; Ar represent an aromatic ring; R1 is the same or different and represents a substituted or unsubatituted monovalent hydrocarbon group, a hydroxyl group, an alkoxy group, a nitro group, a cyano group or a halogen atom; R2 and R3 each represents a hydrogen atom and a methyl group; R4 is the same or different and represents a substituted or unsubstituted monovalent hydrocarbon group; R5 represents a substituted or unsubstituted pyridinium group; a represents an integer of 0 to 2; and b represents 2 to 3;
(c) a nitrile rubber having carboxyl groups, and a carboxyl terminated butadiene acrylonitrile is preferred; and
(d) an inorganic filler, and an aluminum hydroxide is preferred.
wherein the weight ratio of the accelerator to the epoxy resin is between 0.5:1 to 0.1:1 0.008:1 to 0.03:1, the weight ratio of the nitrile rubber to the epoxy resin is between 1:5 to 10:1 and the weight ratio of the inorganic filler to the epoxy resin is between 1:2 to 2:1.
The molecular structure of the epoxy resin in the component (a) is unlimited only if that the structure contains two or more epoxy groups in every monomer. The epoxy resin of the present invention can be used either individually or as a combination of the two or more kinds of epoxy resin. Moreover, the component (a) of the present invention is (1) a curable epoxy resin which is liquid at ordinary temperature; or (2) a curable liquid mixture of liquid curable epoxy resin and solid curable epoxy resin at ordinary temperature and the latter (i.e. solid curable epoxy resin) can exhibit a liquid state at ordinary temperature by diluting it with a diluents. Herein, xe2x80x9cliquid at ordinary temperaturexe2x80x9d means that the above epoxy resin has fluidity between 25xc2x0 C. and 40xc2x0 C. and a so-called semisolid state is included.
The curable epoxy resin (1) is liquid at ordinary temperature comprising: a bisphenol A type epoxy resin having an average molecular weight of about 500 or less; a bisphenol F type epoxy resin; a phenol novolak type epoxy resin having an average molecular weight of about 570 or less; an alicyclic epoxy resin such as 1,2-epoxyethyl-3,4-epoxycyclohexane, 3,4-epoxycyclohexylcarboxylic acid- 3,4-epoxycyclohexylmethyl and bis(3,4-epoxy-6-methylcyclohexylmethyl) adipate; a glycidyl ester epoxy resin such as diglycidyl hexahydrophthalate, diglycidyl 3-methyl-hexahydrophthalate and diglycldyl hexahydroterephthalate; a glycidylamine type epoxy resin such as diglycidyl aniline, diglycidy toluidine, triglycidyl-p-ammophenol, tetraglycidyl-m-xylylenediamine and tetraglycidyl bis(amino-methyl) cyclohexane; and a hydantoin type epoxy resin such as 1,3-diglycidyl-5-methyl-5-ethylhydantoin.
The curable epoxy resin (2) is a mixture of liquid curable epoxy resin and solid curable epoxy resin at ordinary temperature, wherein the liquid curable epoxy resin has been described in above paragraph. The solid curable epoxy resin such as a bisphenol F type epoxy resin, a novolak type epoxy resin, an alicyclic epoxy resin, a glycidyl ester type epoxy resin, a glycidylamine type epoxy resin, a triazine type epoxy resin and a hydantoin type epoxy resin, can be used in this present invention after being diluted with a diluent. Herein, any diluent, which can dissolve or disperse the epoxy resin and maintain its fluidity, can be used in the present invention.
Component (b) is represented by formula (I) or (II): 
wherein E, Ar, R1, R2, R3, R4, R5, a and b have the same meanings as defined above. It needs to be emphasized that the component (b) of formula (I) is an onium hexafluoroantimonate having an aromatic ring Ar bonded to a sulfur, nitrogen or phosphorous atom through xe2x80x94(R2R3)xe2x80x94. The aromatic ring Ar is a benzene ring, a naphthalene ring, an anthracene ring or a pyrene ring. A benzene ring is preferred since it can be synthesized more easily.
The aromatic ring Ar is unsubstituted or substituted by one or two R1(s). R1 group is selected from the group consisting of a straight or branched alkyl group such as methyl, ethyl, propyl, butyl, pentyl, octyl, decyl, dodecyl and tetradecyl; a cycloalkyl group such as cyclohexyl; a aryl group such as phenyl and naphthyl; alkaryl group such as tolyl and xylyl; an aralkyl group such as benzyl and 2-phenyl-ethyl; an alkenyl group such as vinyl, allyl and butenyl; a monovalent substituted hydrocarbon group such as chloromethyl; a hydroxyl group; a alkoxyl group such as methoxy, ethoxy, propoxy and t-butoxy; a nitro group; a cyano group; and a halogen atom, such as fluorine, chlorine, bromine and iodine. When a phosphonium salt and a pyridinium salt having relatively low activity are used, it is preferred that R1 is an electron-withdrawing group or a halogen atom, since a high curing rate can be thus obtained. The introduced position of R1 group is optional. However, when the Ar is a benzene ring, the R1 position is preferably 2-position and/or 4-position since the compound can be synthesized easily. When R1 is a bulky group, the position is particularly preferably 4-position since there is no lowering of activity caused by steric hindrance.
R2 and R3 groups are independently a hydrogen atom or a methyl group and preferably hydrogen atoms since the compound can be synthesized easily. When catalytic activity must be increased, it is preferred that at least one of R2 and R3 is a methyl group.
The R4 group is selected from the group consisting of a straight or branched alkyl group such as methyl, ethyl, propyl, butyl, pentyl, octyl, decyl, dodecyl and tetradecyl; a cycloalkyl group such as cyclohexyl; an aryl group such as phenyl and naphthyl; an alkaryl group such as tolyl and xylyl; an aralkyl group such as benzyl and 2-phenyl-ethyl; an alkenyl group such as vinyl, allyl and butenyl; a monovalent substituted hydrocarbon group such as hydroxyphenyl, methoxyphenyl, ethoxyphenyl, cyanophenyl, chlorophenyl, acetoxyphenyl, propanoylphenyl, methoxvcarbonylphenyl and ethoxycarbonylphenyl.
The R5 is a substituted or unsubstituted pyridinium group such as pyridinium, 2- or 4- methylpyridiniumn, 2,4-dimethylpyridinium, 2- or 4- cyanopyridinium, 2- or 4- methoxycarbonylpyridinium and 2- or 4- ethoxycarbonylpyridinium. It is preferred that R5 has a nucleophilic group such as a cyano group at 2-position or 4- position.
The component (b) is a sulfonium salt, much as methyl (4-methoxybenzyl) (1-naphthylmethyl) sulfonium hexafluoroantimonate, methyl (4-hydroxyphenyl) benzyl sulfonium hexafluoroanzimonate, dimethylbenzyl sulfonium hexafluoroantimonate, dimethyl(4-methylbenzyl) sulfonium hexafluoroantimonate, dimethyl(4-methoxybenzyl) sulfonium hexafluoroantimonate, dimethyl(4-ethoylbenzyl) sulfonium hexafluoroantimonate, dimethyl(4-t-butoxybenzyl) sulfonium hexafluoroantimonate, dimethyl(4-nitrobenzyl) sulfonium hexafluoroantimonate, dimethyl(4-cyanobenzyl) sulfonium hexafluoroantimonate, dimethyl(4-chlorobenzyl) sulfonium hexafluoroantimonate, methylphenylbenzyl sulfonium hexafluoroantimonate, (4-hydroxyphenyl)benzyl sulfonium hexafluoroantimonate, dimethyl(1-naphthylmethyl) sulfonium hexafluoroantimonate, methyl (4-hydroxyl) (1-naphthyrlmethyl) sulfonium hexafluoroantimonate, dimethyl(xcex1-methylbenzyl) sulfonium hexafluoroantimonate, etc.; an ammonium salt such as dimethylbenzyl(4-methoxybenxyl)ammonium hexafluoroantimonate, trimethylbenzylammonium hexafluoroantimonate, dimethylphenylbenzyl ammonium hexafluoroantimonate, dimethylphenyl(4-nitrobenzyl) ammonium hexafluoroantimonate, (4-cyanobenzyl) ammonium hexafluoroantimonate, (4-chlorobenzyl) ammonium hexafluoroantimonate, etc.; a phosphonium, salt such as triphenylbenzyl phosphonium hexafluoroantimonate, triphenyl(4-nitrobenzyl) phosphonium hexafluoroantimonate, etc.; and a pyridinium salt such as (4-methylbenzyl)-4-cyanopyridimum hexafluoroantimonate, (4-t-butylbenzyl)-4-cyanopyridinium hexafluoroantimonate, (4-methoxybenzyl)-4-cyanopyridinium hexafluoroantimonate, (4-chlorobenzyl)-4-cyanopyridinium hexafluoroantimonate, (xcex1-methylbenzyl)-4-cyanopyridinium hexafluoroantimonate, benzyl-2-cyanopyridinium hexafluoroantimonate, (xcex1-methylbenzyl)-2-cyanopyridinium hexafluoroantimonate, 4-methoxycarbonylpyridinium hexafluoroantimonate, etc.
The component (c) is a nitrile rubber containing carboxyl groups, which is a copolymeric rubber of an acrylonitrile and a butadiene and carboxylated at the molecular chain terminal. The carboxyl group of the carboxyl-containing nitrile rubber is in the range from 2 to 8% by weight. For example, the component (c) of the present invention is a carboxyl terminated butadiene acrylonitrile.
The component (d) of the present invention is an inorganic filler to make shrinkage small during the curing process, comprising a pulverized silica, spherical silica, fumy silica and precipitated silica; and powders of alumina, titania, zirconia, antimony oxide, aluminum hydroxide, calcium carbonate, glass beads, boron nitride, aluminum carbide, aluminum nitride, silicon carbide, silicon nitride, silicon nitrocarbide, titanium carbide, titanium nitride, etc.
The mixed ratio of the component (c) to component (a) is in the range from 1/5 to 10/1. If the ratio above-mentioned is greater than 10/1, the heat-resistant property of the adhesive composition will be decreased; however, if the ratio is smaller than 1/5, the adhesion strength of the adhesive composition will also be decreased. Therefore, the preferred ratio of the component (c) to component (a) of the present invention is in the range from 1/2 to 2/1. In addition, the mixed ratio of the component (b) to component (a) is in the range from 0.05% to 10% by weight. If the ratio of the component (b) to component (a) is smaller than 0.05% by weight, the curing rate of the adhesive composition will be decreased; however, if it is greater than 10% by weight, the storage stability of the adhesive composition will be decreased and the poor physical property is obtained after the adhesive composition is cured. Therefore, the preferred ratio of the component (b) to component (a) of the present invention is in the range from 0.8% to 3% by weight.
There is no curing agent contained in the adhesive composition of the present invention, such as a novolak type epoxy resin, amine or anhydride compounds. Therefore, the disadvantages caused by the curing agent, such as uniformity, toxicity and moisture absorption, is avoided. Moreover, the present invention can further comprise a flame retardant, silane compounds or titanium coupling agent, pigment, dye, etc.
The components used in the following examples and comparative examples are as follows:
First, the components (a), (c) and (d) are mixed and a solvent, such as methyl ethyl ketone, is added into the above mixture. Then, the mixture is pulverized in a pulverizer and a solution with formula (I) is formed. The component (b) is mixed with methyl ethyl ketone and then a solution with formula (II) is formed. The solutions with formula (I) and formula (I) are mixed and pulverized by a pulverizer and then an adhesive solution with formula (III) is formed.
The viscosity test of the adhesive solution is carried out in a condition of Brookfied (LVT type). The period, for which the viscosity increases to twice the original one when the adhesive solution is standed at room temperature, is called a pot life. Moreover, the glass transition temperature of the adhesive solution is measured with Du Pont TMA 2940.